Blown-film apparatus is used to manufacture plastic bags and the like. A molten tube of plastic is extruded from an annular die and then stretched and expanded to a larger diameter and a reduced thickness by the action of overhead nip rolls and internal air pressure. Where the film is to be wound in a roll, the annular die or the overhead nip rolls are slowly rotated to distribute film thickness irregularities caused by die imperfections. To control the circumference of the finished tube, it is generally necessary to adjust the volume of air captured inside the tube between the annular die and the overhead nip rolls. It has been conventional to adjust the entrapped volume of air by operating valves in the small inflation line connected to the tube interior. These valves were opened or closed in what is known as "bang-bang" control in response to measurements of tube size. E.g., Uemura et al. U.S. Pat. No. 3,932,080, Suh et al. U.S. Pat. No. 3,159,698, and Hearns et al. U.S. Pat. No. 3,700,370.
To increase production speeds of blown film lines, a continuous stream of cooling air has been admitted through passages in the die, directed against the inner wall of the tube and removed through passages in the die. In such dies, in order to regulate the flow rate in a dynamic manner a mechanical feeler that follows the wall of the tube has been used. Schott U.S. Pat. No. 3,980,418, for instance, shows a single feeler mechanically connected to a pneumatic regulator valve which, through a pneumatic cylinder, proportionally controls a flapper valve in the internal air supply line.
The copending application of Schott, Ser. No. 964,983, filed Nov. 30, 1978, entitled "Optical Control of Tubular Film Size," discloses an optical control system for varying the flow of internal cooling air to maintain tube circumference. An electrical signal representative of changes in tube circumference from a set point is generated by one or more optical sensors. The electrical signal controls the position of a valve which, in turn, determines the rate of cooling air flow.